FIG. 1A illustrates a wind turbine 1, comprising a wind turbine tower 2 on which a wind turbine nacelle 3 is mounted. At least one rotor blade 5 is attached to a hub 6 to form the rotor. Each blade can rotate about its own longitudinal axis. This is called blade pitching. The hub 6 is connected to the nacelle 3 through a low speed shaft (not shown) extending from the nacelle front. The axis or rotation of the rotor is along the low speed shaft. The wind turbine illustrated in FIG. 1A may be a small model intended for domestic or light utility usage, or may be a large model, such as those that are suitable for use in large scale electricity generation on a wind farm. In the latter case, the diameter of the rotor may be as large as 150 meters or more.
The rotor blades of wind turbines are designed to extract power from the wind by virtue of their aerodynamic shape, and subsequent wind induced rotation. For horizontal axis wind turbines, the rotation of the rotor about its axis turns a drive shaft connected in turn to a generator which produces electricity. A low speed drive shaft may be used, coupled to a high speed shaft, or alternatively a direct drive shaft may be used. For horizontal axis wind turbines to operate efficiently and extract the maximum power from the wind, the wind turbine nacelle is rotated to make the rotor face the wind to the greatest extent possible, such that the rotational axis of the rotor is aligned with the wind direction.
Wind turbines, and in particular larger wind turbines, will have a system for rotating the nacelle such that the rotor is oriented in a wind direction. These systems are commonly known as yaw systems, or azimuth drives, and allow a wind turbine to continue to extract maximum energy from oncoming winds, despite changes in wind direction. A purpose of the yaw system is therefore to correctly orient the rotor to the correct yaw angle relative to the prevailing wind direction so as to extract the optimum amount of energy from the wind. The yaw system is usually located between the wind turbine tower and the nacelle and typically comprises a bearing that is fully rotatable around an axis co-linear with the tower, and one or more electric or hydraulic drives for rotating the bearing relative to the tower. In this way, the nacelle, mounted on the bearing, can be turned through 360 degrees in the horizontal plane.
Many different yaw systems are known, and often comprise a number of components integrated in part in the nacelle and in part in the top of the turbine tower. The overall system for wind direction tracking might comprise an azimuth bearing, yaw drive, yaw brakes, a locking device, and a control system. The azimuth bearing allows the nacelle to rotate relative to the turbine tower. The yaw drive is coupled to the bearing via a gearing system and provides the force to rotate the nacelle about the bearing. Hydraulic or electric drive systems are widely used. The yaw brakes absorb the yawing moment after a completed yawing operation and are required unless the yaw drive has an integrated braking function. A locking device is commonly used in larger turbines so that the yaw drive is positively locked in place. The control system provides the operating logic required to automatically position the rotor blades into the wind. For the avoidance of doubt, the present invention can be used with any type of yaw system.
Since it is desirable, in upwind turbines, for the turbine rotor to face directly into oncoming wind at all times during operation to extract maximum energy, it is useful to define the yaw error and yaw angle. The yaw error is the angular difference between the direction of the wind, and the direction in which rotor is facing. FIG. 1B shows some internal components of a typical wind turbine, using the same references as FIG. 1A, including the yaw drive system 20 and yaw motor 21. FIG. 1B shows how the blades 5 can pitch about their longitudinal axis, and how the nacelle can yaw about the axis of the turbine tower. The direction in which the rotor is facing can be considered as the direction in which the nacelle is pointing, or the direction in which the axis of rotation of the rotor is pointing, since they are the same. When the yaw error is zero or substantially zero, the blades are considered to be facing the wind direction. The yaw angle can be defined as the angular difference between a 0° point, usually defined by a geographical direction such as north, and the direction in which the rotor is facing.
The purpose of the control system is to ensure that the yaw error is as small as possible, whilst also ensuring that change in yaw angle to correct yaw error is not too sensitive to avoid continuous small yaw movements which would result in premature wear of the mechanical components. Various operating methods are possible to try to ensure such a compromise, but they will typically involve the measuring of a mean value of wind direction using a wind sensor and a comparison with the azimuth position of the nacelle to determine the yaw error. If the yaw error exceeds a particular threshold then the yaw system is activated to change the yaw angle to reduce the yaw error.
Known yaw control systems rely on accurate wind direction measurements, which are usually performed by wind vanes or other instruments such as ultrasonic anemometers. In addition, wind direction instruments are often mounted on the nacelle of the turbine in a region of airflow that is directly affected by the rotor blades which may result in an incorrect reading. We have appreciated that there is a need for an improved system for determining and controlling yaw error to maximise energy capture and reduce fatigue loads.